Antioxidant defenses and non-specific immunity at enzymatic and transcriptional levels in response to dietary carbohydrate in a typical carnivorous fish, hybrid grouper (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂)

The present study was conducted to explore the influence of dietary carbohydrate on antioxidant capacity and non-specific immunity of hybrid grouper, which would contribute to determine the tolerable dietary carbohydrate content. Seven diets with grade levels of carbohydrate (5.27, 8.95, 11.49, 14.37, 17.78, 20.82 and 23.65%) were fed to triplicate groups of fish for 10 weeks. Results showed that the inclusion of carbohydrate above 11.49% produced significant increased content of hydrogen peroxide (H₂O₂) in liver and malondialdehyde (MDA) in both serum and liver. The specific activity of catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (Gpx) and total antioxidative capacity (T-AOC) were significantly elevated with the increase of dietary carbohydrate from 8.95 to 23.65%, which may be associated with the reduced hepatic soluble protein content. However, opposite variation was observed in the expression of antioxidant related genes (SOD1 and Gpx), which was partly caused by the activation of NF-E2-related factor 2 (Nrf2) and inhibition of Kelch-like-ECH-associated protein 1 (Keap1) at the transcriptional level. The immunoglobulin M (lgM) content and activity of lysozyme and CCP in serum significantly depressed when dietary carbohydrate was above 11.49%. The expression of pro-inflammatory cytokines (TNF-α, IL-1β and IL-8) was significantly increased with the increase of dietary carbohydrate from 5.27 to 8.95% and thereafter significantly reduced, which was consistent with the changed expression of toll-like receptor 2 (TLR2) and nuclear factor κΒ (NF‐κΒ). In above, high dietary carbohydrate significantly impaired the antioxidant capacity and reduced the non-specific immunity of hybrid grouper, and the tolerable dietary carbohydrate content should not exceed 11.49%.